1. Field of the Invention
The present invention relates to a method and system for obtaining positioning data of moving objects, for example artillery munitions, and in particular, for improving the acquisition time of GPS signals, including but not limited to, compensating corrupted GPS signals.
2. Background
Methods of precisely locating and controlling rocket-propelled weapons, such as the tomahawk missile, are well known in the art. In a first method, the rocket-propelled weapon is steered from the launch location. After launch, the location of the weapon is known by either sophisticated radar or by placing a positioning device on the weapon. The positioning device relays the weapon's position back to the launch location. The positioning data includes such data as longitude, latitude, and altitude. With this positioning data, the operator at the launch location can steer the weapon to the desired location.
A second method involves placing a computer and a positioning device, such as GPS, inside of the rocket-propelled weapon. Then a predetermined strike location is entered into the on-board computer. After launch, the weapon determines its position from the positioning device. This positioning information is then fed to the on board computer where it is compared with the predetermined strike location. After the computer makes the position comparison, it sends information to the ailerons, elevator, and rudder of the weapon to steer it to the predetermined strike location.
Another method involves programming the topology of the land around a predetermined strike location into the on-board computer. After the launch, the rocket-propelled weapon uses an on-board camera to identify the topography of the land. This visual information is then sent to the computer and compared with the pre-programmed topology information. After the computer makes the topology comparison, it sends information to the ailerons, elevator, and rudder of the weapon to steer it to the predetermined strike location.
In an extension of the concept above, the military is interested in obtaining a precise position of artillery shells. However, using the technologies described above is very costly. This is due to the cost of placing a computer and/or a position-locating device, such as GPS, on board an artillery munition.
GPS is a Department of Defense developed, worldwide, satellite-based radio-navigation system. It will provide primary radio-navigation data well into the next century. The GPS satellite constellation consists of 24 satellites providing two levels of service, Standard Positioning Service, and the Precise Positioning Service.
The Standard Positioning Service (SPS) makes available the GPS L1 frequency, which contains the coarse acquisition (C/A) code and navigation data message. The L1 navigation message contains clock corrections; ephemeris data; satellite health and accuracy; and the identification of the satellite (Almanac data). The SPS is available to all GPS users on a continuous, worldwide basis.
The Precise Positioning Service (PPS) provides accurate military positioning service available to authorized users only. The PPS is provided on the GPS L1 and L2 frequencies. The design of PPS was primarily for U.S. military. P(Y) code capable military users can obtain positioning data much more accurate than that of the C/A code users.
Each of the 24 satellites transmit on two L-band frequencies: L1=1575.42 MHz and L2=1227.6 MHz. The system uses three pseudo-random noise (PRN) ranging codes. The C/A code has a 1.023 MHz chip rate (a period of one millisecond (ms)) and its primary use is to acquire the P-code. The precision (P) code has a 10.23 MHz rate, a period of 37 weeks, restarted every week at the Saturday/Sunday boundary, and is the principal navigation ranging code. The Y-code is used in place of the P-code whenever PPS capability is to be denied to unauthorized users. Each satellite transmits a navigation message containing its orbital elements, clock behavior, and system time and status messages. In addition, almanac data is provided which gives satellite specific data for each active satellite.
An alternative method of tracking munitions involves attaching an RF translator onto an artillery shell. The RF translator receives a GPS signal, as is known in the art, converts it to an S band signal (2266.5 MHz), then transmits the S band signal to a ground antenna at a ground station. The S band conversion is used so that the receiver does not interpret a GPS signal coming from a satellite as one coming from the artillery shell. Furthermore, the conversion to the S band "freezes" the GPS navigation data in time at the exact location of the muniton. The ground antenna then detects the pilot tone identifying the S band signal. Subsequently, the ground station receives the S band signal and converts it back to a GPS signal. The receiver then processes the GPS signal, gathering the navigation data contained on those satellites. Thus, the ground station will know the continuously updated position of the artillery shell up to impact.
The artillery position identifying technique provides an inexpensive alternative; however, problems surface with operation of the system. When the S band signal is transmitted to a ground antenna, it induces two effects, which work against fast GPS signal detection and acquisition difficult. First, the motion of the munition in relation to the ground antenna creates a Doppler effect affecting the code rate. This can slow detection and acquisition of the carrier of the translated GPS signal from the munition. Second, the code position is affected by the time delay created when the translated signal travels from the satellite to the munition to the ground receiver, rather than the normal operating signal traveling from the satellite to the receiver. This can slow acquisition of the GPS information carried in the translated GPS signal from the munition, and thus slow extraction of the pertinent navigation data of the munition.
Because the artillery shell is in the air for a short time (&lt;120 seconds), fast acquisition time is necessary. For fast direct P(Y) acquisitions, the total search area (code phase and carrier frequency errors) must be small. This requires that the pre-positioning data used by the base station (the nominal code phase and nominal carrier frequency, which the receiver uses to assess true code phase and carrier frequency) must be accurate as well.
The S band data link effects influence both the code phase and carrier frequency. In order to perform fast direct P(Y) acquisitions, the receiver should correctly compensate for the S band data link effects.
The carrier Doppler is determined from the equation: Doppler=(Frequency*Velocity)/c (c=Speed of Light). For an artillery shell-positioning program, the maximum velocity of the artillery shell is around 800 m/s. This translates to 800/3.times.10.sup.8 =2.7 ppm (parts per million or parts per Megahertz) uncertainty. This uncertainty is 54 times larger than the local clock error (worst case local clock drift is 0.05 ppm). Therefore, the carrier Doppler correction is necessary, because stretching or compression of the carrier frequency will have a proportional effect on the code frequency (e.g., the code rate).
Code phase is determined by time in the GPS week. To properly acquire a GPS signal the code phase of the satellite and the code phase of the receiver must by synchronized. The time delay caused by the S band data link changes the nominal code phase for an acquisition. The artillery shell will have traveled approximately 3.25 to 4.25 seconds (250 ms launch pulse received, 1 sec pilot tone detect, and 2-3 sec for software initialization and noise measurements (See FIG. 6)) before acquisitions can start. Assuming an average velocity of 600 m/sec, the artillery shell will travel approximately 1950 m to 2550m before acquisitions. Converting this distance to p chips (1 p chip.apprxeq.29.3 m) yields a range of 66 to 87 p chips and is significant given the goal of fast searches. The significance stems from the fact that the code phase of the receiver and the code phase of the satellite must correlate to acquire the GPS signal. A typical receiver searches at a rate of 50 chips/second using two taps. Therefore, if the code phase is off by 66-87 p chips, an extra 1.32-1.74 seconds will elapse before acquisition. The code position should be corrected to compensate for the time delay caused by the travel of the translated signal.
The pre-positioning software needs to add an extra term, carrier frequency Doppler, into the determination of the nominal carrier frequency. The determination of the carrier Doppler stems from position and velocity estimating data (e.g., trajectory model data, radar tracking data or inertial tracking data). The position and velocity estimating data is used to improve acquisition performance. The model provides the GPS ground receiver software with estimated positions of the munition until the ground receiver can acquire a position of the munition itself through the S band data.
The position and velocity data is given for specific time increments, such as milliseconds. The position estimating data is then inputted into the GPS receiver. It should be noted that the velocity estimating data either can be inputted into the receiver or can be derived from the position estimating data. Thus, the receiver will have an estimate of the velocity of the munition at specific periods. Using this velocity estimating data, the GPS software can calculate the new code rate. The use of the velocity term in the position and velocity estimating data is discussed more in the detailed description.
The pre-positioning compensation for the time delay is determined from the position and velocity estimating data also. The position and velocity estimating data informs the receiver approximately, where the munition should be at a specific time after launch. Range from the munition to the ground station is then computed, based upon the known ground station location. This common mode range term is added to the receiver to calculate the time delay created in the S band signal. By knowing the time delay the receiver can adjust its code phase to align with the code phase of the satellite. Thus, the code phase acquisition time is reduced. A further discussion of the time delay is in the detailed description.
Therefore, it is an object of this invention to provide a device to improve the performance speed of search and probability of detection of GPS acquisitions in munitions.
A further object of this invention is to provide new compensation terms in the pre-positioning data in determination of the nominal carrier frequency and the nominal code phase.
A further object of this invention is to provide a carrier frequency Doppler term which compensates for the carrier Doppler effect due to the relative velocity of the munition with respect to the GPS receiver.
A further object of this invention is to provide a common mode range term, which compensates for the time delay effect created by S band translation.
A further object of this invention to provide a method to improve the speed of, and probability of, GPS acquisitions for munitions.
A further object of this invention is to provide a method of inserting new compensation terms in the pre-positioning data to determine of the nominal carrier frequency and the nominal code phase.
A further object of this invention is to provide a method to insert a common mode Doppler term which compensates for the carrier Doppler effect due to the velocity of the munition with respect to the GPS receiver.
A further object of this invention is to provide a method to insert a common mode range term, which compensates for the time delay effect, created by the S band translation.
A further object of this invention is to provide accurate positioning data for munitions to allow future munitions to hit their targets accurately.
A further object of this invention is to insert position and velocity estimating data of a munition into the GPS ground receiver to assist the receiver in finding the munition faster.
A further object of this invention is to use position and velocity estimating data from the munition to create a carrier Doppler compensation term to use in the pre-positioning software of the GPS ground receiver.
A further object of this invention is to use position and velocity-estimating data from the munition to create a common mode range compensation term to use in the pre-positioning software of the GPS ground receiver.